A new and highly sensitive method for measuring 3-methoxytyramine using HPLC with electrochemical detection. Studies with drugs which alter dopamine metabolism in the brain.
3-Methoxytyramine (3-MT) is a minor metabolite of dopamine which is suggested to reflect the turnover and utilization of dopamine. A novel, isocratic HPLC method has been developed which can be used to analyse 3-MT in homogenates of rat brain without the need for additional purification procedures. Furthermore, the coulometric electrochemical detection system is sensitive enough to measure 3 pg of 3-MT (equivalent to 0.6 ng/g tissue wet weight). 3-Methoxytyramine was measured in the striatum and n. accumbens after decapitation and rapid freezing, using 3-methoxy-4-hydroxybenzylamine as the internal standard. The effects of dopaminergic and other drugs on this metabolite were examined using this method. alpha-Methyl-p-tyrosine (200 mg/kg i.v.) produced parallel linear decreases in dopamine and 3-MT in naive rats, but not those pretreated with tranylcypromine (5 mg/kg i.p.). Methamphetamine (0.3-10 mg/kg i.p.) and amphetamine (0.3-10 mg/kg i.p.) both dose-dependently increased 3-MT in naive and tranylcypromine-pretreated rats. In naive animals, 3-MT was not altered by intraperitoneal injection of the dopamine reuptake inhibitors, bupropion (10 mg/kg) and nomifensine (10 mg/kg) or by sibutramine HCl (3 mg/kg), amitriptyline (10 mg/kg), desipramine (10 mg/kg) and zimeldine (10 mg/kg). 3-Methoxytyramine was decreased by apomorphine (5 mg/kg i.p.) and also by large doses of the selective D2 antagonist, BRL 34778 (5 mg/kg i.p.) or L-DOPA (50 mg/kg i.p.). The selective D1 antagonist, SCH 23390 (0.1 or 5 mg/kg i.p.) was without effect. In tranylcypromine-pretreated rats, 3-MT was dose-dependently reduced and increased by apomorphine (0.01-5 mg/kg i.p.) and BRL 34778 (0.1-5 mg/kg i.p.), respectively. The drug SCH 23390 (0.1-5 mg/kg i.p.) produced much smaller increases in 3-MT which were probably mediated through the striatonigral pathway. Overall, the data suggest that measurement of 3-MT, after inhibition of monoamine oxidase, is a useful index of the release and utilization of dopamine. However, after substantial and prolonged depletion of dopamine, levels of 3-MT in naive animals are a better index. Also, the formation of 3-MT in naive rats rats provides a sensitive method for distinguishing between dopamine releasing agents and reuptake inhibitors.
Heal
D J
DJ
Boots Pharmaceuticals Research Department, Nottingham, U.K.
Frankland
A T
AT
Buckett
W R
WR
D000818:Animals; D000928:Antidepressive Agents / Q000494:pharmacology; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003342:Corpus Striatum / Q000378:metabolism; D003913:Dextroamphetamine / Q000494:pharmacology*; D004298:Dopamine / Q000031:analogs & derivatives* / Q000032:analysis / Q000378:metabolism*; D018492:Dopamine Antagonists; D004563:Electrochemistry / Q000379:methods; D007980:Levodopa / Q000494:pharmacology; D008297:Male; D008694:Methamphetamine / Q000494:pharmacology; D008781:Methyltyrosines / Q000494:pharmacology; D009714:Nucleus Accumbens / Q000378:metabolism; D010293:Pargyline / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014191:Tranylcypromine / Q000494:pharmacology; D019805:alpha-Methyltyrosine
D000928:Antidepressive Agents; D018492:Dopamine Antagonists; D008781:Methyltyrosines; D014191:Tranylcypromine; D008694:Methamphetamine; D007980:Levodopa; D019805:alpha-Methyltyrosine; D010293:Pargyline; C001746:3-methoxytyramine; D003913:Dextroamphetamine; D004298:Dopamine